The invention is based on a pressure switch valve as generically defined hereinafter.
In brake systems with an apparatus for limiting drive slip or wheel spin, also known as traction control, it is desirable when the traction control comes into play to have the brake linings of the wheel brakes engage the brake disk or drum as fast as possible. Contrarily, when brake pressure is next fed to the wheel brake or brakes of the spinning driven vehicle wheel or wheels, the brake pressure should rise at a defined speed. Better control of brake pressure metering can be attained in this way.
For the above reason, a hydraulic brake system for vehicles with an apparatus for limiting drive slip at the drivable vehicle wheels, has been proposed which has a pressure generator that can be acted upon hydraulically and operates by the plunger principle, to generate brake pressure (German Patent Application P 39 36 578.6). The pressure generator, connected to a pressure source, is preceded by a pressure switch valve with a throttle that is inoperative until the brake linings contact the drum or disk. The pressure rising in the activated part of the brake system upon brake lining application switches the valve, making the throttle operative and supplying the pressure generator with a reduced quantity of pressure fluid. The next time the brake pressure is increased, the result is a reduced speed of the pressure rise.
A hydraulic brake system with an apparatus for limiting drive slip has also been proposed that has a high-pressure pump to generate brake pressure in the wheel brake of a spinning vehicle wheel and a low-pressure pump, called a precharging pump, for supplying the high-pressure pump with pressure fluid (German Patent Application P 40 17 872.2). The low-pressure pump is connected in the brake system in such a way that as the traction control begins to become operative it first feeds directly into the wheel brake, causing the brake lining to contact the drum or disk, before pumping by the high-pressure pump begins. However, under extreme conditions, the pressure generated by the low-pressure pump is not always adequate for application of the brake lining. A pressure switch valve that is intended to limit the pressure generated by the pump is disposed parallel to the low-pressure pump.